Numerous proposals are known from the prior art to provide olefin polymerization catalysts by combining a solid component comprising magnesium, titanium and chlorine with an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminum compound.
For convenience of reference, the solid titanium-containing constituent of such catalysts is referred to herein as "procatalyst", the organoaluminum compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA).
The catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer. However, further improvements in productivity at high stereoregularity are still being sought.
Supported coordination catalysts are disclosed in numerous patents, such as U.S. Pat. Nos. 3,953,414, 4,115,319, 4,224,181 and 4,329,253.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of polyolefins in such high yield as to obviate the necessity of extracting residual catalyst components in a deashing step. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic or otherwise stereoregular products to obviate the necessity of extracting atactic polymer components.
Although many chemical combinations provide active coordination catalyst systems, practical considerations have led the workers in the art to concentrate on certain preferred components. The procatalysts typically comprise as dominant and essential constituents magnesium, titanium (generally in tetravalent form), chlorine and as electron donor an aromatic ester such as ethyl benzoate or ethyl-p-toluate. The cocatalyst typically is an aluminum trialkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with selectivity control agent. The selectivity control agent typically is an aromatic estr such as ethyl-para-methoxybenzoate (ethyl anisate) or methyl-p-toluate.
Among the methods of preparing procatalysts of this type, a distinction may be made between those in which the starting material is a magnesium chloride and those in which magnesium chloride is produced by halogenation of an organic compound in the course of the procatalyst synthesis.
Improved catalysts of the latter type and processes for olefin polymerization with such catalysts are disclosed in U.S. Pat. Nos. 4,329,253, 4,393,182 and European patent application No. 19,330, all by Goodall et al. The catalysts of Goodall et al employ as procatalyst a composition prepared by halogenating a magnesium compound MgR'R" (wherein R' and R" are alkyl, aryl, alkoxide or aryloxide groups and R" may also be a halogen) by reaction with a halide of tetravalent titanium in the presence of an electron donor and a halohydrocarbon followed by contact of the halogenated product with a tetravalent titanium compound.
This invention provides still further improvements of the catalysts and processes of the prior art. Catalysts which comprise the solid titanium-containing catalyst constituents of this invention in combination with an organoaluminum cocatalyst and a selectivity control agent or with an at least partial reaction product of an organoaluminum compound and a selectivity control agent are capable of producing polypropylene of commercially desired isotacticity at very high productivity.